FIG. 1 illustrates a borescope 3 being used to examine an internal component 6 in a gas turbine engine 8 which would otherwise be hidden from view. Such a borescope 3 generally includes a tube 9, which supports a lens system including lenses 12, 14 and 16 and also possibly a prism 18 for viewing in a region which lies off the optical axis of the borescope.
Borescopes, during their lifetimes, suffer abuse inflicted by the following factors.
(1) Mechanical shock, which can crack lenses or prisms, dislodge lenses and prisms from their original positions, or scratch the lenses and prisms. PA1 (2) Fogging of optical elements of the borescope resulting from PA1 (3) Mechanical deformation which deforms the borescope tube 9 and misaligns the optical elements.
(a) outgased vapors released by the adhesives used to hold the optical elements in position when the adhesives are subject to overheating; PA2 (b) degradation of anti-reflective coatings on the optical elements caused by excessive temperature or excessive radiation; and PA2 (c) contamination of the optical elements by accidental spillage of solvents or other contaminants.
One or more of the above types of damage can be suffered by a borescope, yet a degradation in the optical performance of the borescope may not be apparent to a human operator. For example, the borescope may be rendered incapable of detecting a defect below a certain size, yet the borescope may appear to be functioning perfectly well.